Lithium silicate glass ceramic and lithium silicate glass comprising a pentavalent metal oxide

ABSTRACT

Lithium silicate glass ceramics and glasses containing specific oxides of pentavalent elements are described which crystallize at low temperatures and are suitable in particular as dental materials.

The invention relates to lithium silicate glass ceramic and glass whichcontain pentavalent metal oxide selected from Nb₂O₅, Ta₂O₅ and mixturesthereof and are particularly suitable for use in dentistry, preferablyfor the preparation of dental restorations.

Lithium silicate glass ceramics are characterized as a rule by very goodmechanical properties, which is why they have been used for a long timein the dental field and there primarily for the preparation of dentalcrowns and small bridges. The known lithium silicate glass ceramicsusually contain as main components SiO₂, Li₂O, Na₂O or K₂O, andnucleating agents such as P₂O₅ as well as additional components such ase.g. La₂O₃.

DE 24 51 121 describes lithium disilicate glass ceramics which containK₂O. They are prepared from corresponding nuclei-containing startingglasses which are heated to temperatures of from 850 to 870° C. for thecrystallization of lithium disilicate. The purpose for which the glassceramics are used is not disclosed.

EP 827 941 describes sinterable lithium disilicate glass ceramics fordental purposes, which also contain K₂O or Na₂O in addition to La₂O₃.The lithium disilicate crystal phase is produced at a temperature of850° C.

Lithium disilicate glass ceramics which likewise contain La₂O₃ as wellas K₂O are known from EP 916 625. A heat treatment is carried out at870° C. for the formation of lithium disilicate.

EP 1 505 041 describes lithium silicate glass ceramics containing K₂O,which, when lithium metasilicate is present as main crystal phase, canbe very satisfactorily machined processed e.g. by means of CAD/CAMprocesses, in order to then be converted by further heat treatment attemperatures of from 830 to 850° C. into high-strength lithiumdisilicate glass ceramics.

EP 1 688 398 describes similar K₂O-containing lithium silicate glassceramics which are moreover substantially free from ZnO. A heattreatment at 830 to 880° C. is applied to them to produce lithiumdisilicate.

U.S. Pat. No. 5,507,981 describes processes for producing dentalrestorations and glass ceramics that can be used in these processes.These are in particular lithium disilicate glass ceramics with a lowlevel of Li₂O which contain as a rule either Na₂O or K₂O.

U.S. Pat. No. 6,455,451 relates to lithium disilicate glass ceramicswhich also contain K₂O in addition to Li₂O. However, the production ofthe desired lithium disilicate crystal phase requires high temperaturesof from 800 to 1000° C.

WO 2008/106958 discloses lithium disilicate glass ceramics for veneeringzirconium oxide ceramics. The glass ceramics contain Na₂O and areproduced by heat treatment of nuclei-containing glasses at 800 to 940°C.

WO 2009/126317 describes GeO₂-containing lithium metasilicate glassceramics which also contain K₂O. The glass ceramics are processed toform dental products primarily by machining.

WO 2011/076422 relates to lithium disilicate glass ceramics which alsocontain K₂O in addition to high levels of ZrO₂ or HfO₂. Thecrystallization of lithium disilicate takes place at high temperaturesof from 800 to 1040° C.

Common to the known lithium disilicate glass ceramics is that theyrequire heat treatments at more than 800° C. in order to effect theprecipitation of lithium disilicate as main crystal phase. A largequantity of energy is therefore also necessary for their preparation.Further, with the known glass ceramics alkali metal oxides, such as inparticular K₂O or Na₂O and also La₂O₃, are as a rule present asessential components which are apparently required for the production ofglass ceramics with the sought properties and in particular theformation of the sought lithium disilicate main crystal phase.

There is therefore a need for lithium silicate glass ceramics during thepreparation of which the crystallization of lithium disilicate can beeffected at lower temperatures. Further, they should also be able to beprepared without the alkali metal oxides, such as in particular K₂O orNa₂O, as well as La₂O₃, previously regarded as necessary, and besuitable in particular for the preparation of dental restorationsprimarily on the basis of their optical and mechanical properties.

This object is achieved by the lithium silicate glass ceramic accordingto any one of claim 1 to 18 or 21. Also a subject of the invention arethe starting glass according to claim 19 or 21, the lithium silicateglass with nuclei according to claim 20 or 21, the process for thepreparation of the glass ceramic and the lithium silicate glass withnuclei according to claim 22 or 23 as well as the use according to claim24 or 25.

The lithium silicate glass ceramic according to the invention ischaracterized in that it comprises pentavalent metal oxide selected fromNb₂O₅, Ta₂O₅ and mixtures thereof and comprises less than 2.0 wt.-% K₂O.

It is preferred that the glass ceramic comprises at least 2.0 and inparticular at least 2.1 wt.-% Nb₂O₅ or Ta₂O₅.

It is further preferred that the glass ceramic comprises the pentavalentmetal oxide or mixtures thereof in an amount of from 0.1 to 8.5, inparticular 1.5 to 8.5 and particularly preferred 2.0 to 8.5 and evenmore preferred 3.5 to 8.5 wt.-%.

It is particularly surprising that the formation of the glass ceramicaccording to the invention with lithium disilicate as main crystal phaseis also achieved in the absence of various components regarded asnecessary for conventional glass ceramics, such as alkali metal oxides,in particular K₂O and Na₂O, even at very low and thus advantageouscrystallization temperatures of in particular from 650 to 750° C. Theglass ceramic also has a combination of optical and mechanicalproperties as well as processing properties that are very advantageousfor the use as dental material.

The glass ceramic according to the invention accordingly preferablycomprises less than 1.0, in particular less than 0.5 wt.-%, preferablyless than 0.1 wt.-% K₂O. It is particularly preferably substantiallyfree from K₂O.

A glass ceramic is also preferred which comprises less than 1.0, inparticular less than 0.5 and preferably less than 0.1 wt.-% Na₂O andparticularly preferred is substantially free from Na₂O.

In a further preferred embodiment, the glass ceramic comprises less than1.0, in particular less than 0.5 and preferably less than 0.1 wt.-%further alkali metal oxide and particularly preferred is substantiallyfree therefrom. The term “further alkali metal oxide” refers to alkalimetal oxide with the exception of Li₂O.

A glass ceramic is also preferred which comprises less than 0.5, inparticular less than 0.1 wt.-% CaO and particularly preferred issubstantially free from CaO.

Further, a glass ceramic which comprises less than 0.1 wt.-% La₂O₃ ispreferred. The glass ceramic is particularly preferably substantiallyfree from La₂O₃.

Further, a glass ceramic is preferred which comprises less than 0.5 andin particular less than 0.1 wt.-% B₂O₃ and particularly preferred issubstantially free from B₂O₃.

A glass ceramic, excluding lithium silicate glass ceramic whichcomprises at least 6.1 wt.-% ZrO₂, is also preferred.

Further, a glass ceramic, excluding lithium silicate glass ceramic whichcomprises at least 8.5 wt.-% transition metal oxide selected from thegroup consisting of oxides of yttrium, oxides of transition metals withan atomic number from 41 to 79 and mixtures of these oxides, is alsopreferred.

The glass ceramic according to the invention preferably comprises 60.0to 85.0, in particular 62.0 to 80.0 and preferably 66.0 to 77.0 wt.-%SiO₂.

It is also preferred that the glass ceramic comprises 11.0 to 21.0 andin particular 12.0 to 20.0 wt.-% Li₂O.

It is further preferred that the molar ratio between SiO₂ and Li₂O isfrom 1.7 to 3.1, in particular 1.75 to 3.0. It is very surprising thatthe production of lithium disilicate is achieved within this broadrange. Specifically at ratios of less than 2.0 conventional materialsusually form lithium metasilicate instead of lithium disilicate.

In a further preferred embodiment the molar ratio between SiO₂ and Li₂Ois at least 2.2, in particular 2.3 to 2.5, and preferably about 2.4, asa glass ceramic with particularly high strength is thus obtained.

The glass ceramic according to the invention can also comprise anucleating agent. P₂O₅ is particularly preferably used for this. Theglass ceramic preferably comprises 0 to 10.0, in particular 2.0 to 9.0and preferably 3.0 to 7.5 wt.-% P₂O₅. Metals such as Ag, Au and Pd canalso be used in amounts of in particular 0.005 to 0.5 wt.-% asnucleating agents.

In a further preferred embodiment, the glass ceramic comprises at leastone and preferably all of the following components:

Component wt.-% SiO₂ 66.0 to 77.0 Li₂O 12.0 to 20.0 pentavalent metal2.0 to 8.5 oxide or mixtures P₂O₅ 0 to 7.0, in particular 3.0 to 7.0Al₂O₃ 0 to 6.0, in particular 3.0 to 4.0.

The glass ceramic according to the invention can moreover also compriseadditional components which are selected in particular from oxides oftrivalent elements, further oxides of tetravalent elements, oxides ofhexavalent elements, melt accelerators, colourants and fluorescentagents. It is, however, preferred that the glass ceramic is free fromarsenic oxide and antimony oxide. These oxides are used in glassceramics for technical applications as agents for homogenizing the melt.As they are potentially damaging to health, they are to be avoided inthe glass ceramic according to the invention as this is used inparticular as dental material.

Suitable oxides of trivalent elements are in particular Al₂O₃, Y₂O₃ andBi₂O₃ and mixtures thereof, and preferably Al₂O₃, already mentionedabove as a component.

The term “further oxides of tetravalent elements” refers to oxides oftetravalent elements with the exception of SiO₂.

Examples of suitable further oxides of tetravalent elements are TiO₂,SnO₂, GeO₂ and ZrO₂, in particular ZrO₂.

Examples of suitable oxides of hexavalent elements are WO₃ and MoO₃.

A glass ceramic which comprises at least one oxide of trivalentelements, at least one further oxide of tetravalent elements and/or atleast one oxide of hexavalent elements is preferred.

Examples of melt accelerators are fluorides.

Examples of colourants and fluorescent agents are oxides of d- andf-elements, such as the oxides of Ti, V, Sc, Mn, Fe, Co, Ta, W, Ce, Pr,Nd, Tb, Er, Dy, Gd, Eu and Yb. Metal colloids, e.g. of Ag, Au and Pd,can also be used as colourants and in addition can also act asnucleating agents. These metal colloids can be formed e.g. by reductionof corresponding oxides, chlorides or nitrates during the melting andcrystallization processes. The metal colloids can be present in theglass ceramic in an amount of from 0.005 to 0.5 wt.-%.

The term “main crystal phase” used below refers to the crystal phasewhich has the highest proportion by volume compared with other crystalphases.

The glass ceramic according to the invention has lithium metasilicate asmain crystal phase in one embodiment. In particular the glass ceramiccomprises more than 5 vol.-%, preferably more than 10 vol.-% andparticularly preferred more than 15 vol.-% lithium metasilicatecrystals, relative to the total glass ceramic.

In a further particularly preferred embodiment, the glass ceramic haslithium disilicate as main crystal phase. In particular the glassceramic comprises more than 10 vol.-%, preferably more than 20 vol.-%and particularly preferred more than 30 vol.-% lithium disilicatecrystals, relative to the total glass ceramic.

The lithium disilicate glass ceramic according to the invention ischaracterized by particularly good mechanical properties and can beproduced e.g. by heat treatment of the lithium metasilicate glassceramic according to the invention. However, it can be formed inparticular by heat treatment of a corresponding starting glass or of acorresponding lithium silicate glass with nuclei.

It has surprisingly been shown that the lithium disilicate glass ceramicaccording to the invention has very good mechanical and opticalproperties and processing properties even in the absence of componentsregarded as essential for conventional glass ceramics. The combinationof its properties even allows it to be used as dental material and inparticular material for the preparation of dental restorations.

The lithium disilicate glass ceramic according to the invention has inparticular a fracture toughness, measured as K_(IC) value, of at least1.6 MPa·m^(0.5) and in particular more than approximately 2.4MPa·m^(0.5). This value was determined using the Vickers method andcalculated using Niihara's equation.

The invention also relates to a lithium silicate glass with nuclei thatare suitable for forming lithium metasilicate and/or lithium disilicatecrystals, wherein the glass comprises the components of theabove-described glass ceramics according to the invention. Thus, thisglass is characterized in that it comprises pentavalent metal oxideselected from Nb₂O₅, Ta₂O₅ and mixtures thereof and comprises less than2.0 wt.-% K₂O. In respect of preferred embodiments of this glassreference is made to the preferred embodiments described above of theglass ceramics according to the invention.

The glass with nuclei according to the invention can be produced by heattreatment of a correspondingly composed starting glass according to theinvention. The lithium metasilicate glass ceramic according to theinvention can then be formed by a further heat treatment, and in turn beconverted into the lithium disilicate glass ceramic according to theinvention by further heat treatment, or the lithium disilicate glassceramic according to the invention can also preferably be formeddirectly from the glass with nuclei. The starting glass, the glass withnuclei and the lithium metasilicate glass ceramic can consequently beregarded as precursors to the production of the high-strength lithiumdisilicate glass ceramic.

The glass ceramics according to the invention and the glasses accordingto the invention are present in particular in the form of powders,granular material or blanks, e.g. monolithic blanks, such as platelets,cuboids or cylinders, or powder green compacts, in unsintered, partlysintered or densely-sintered form. They can easily be further processedin these forms. They can, however, also be present in the form of dentalrestorations, such as inlays, onlays, crowns, veneers, facets orabutments.

The invention also relates to a process for the preparation of the glassceramic according to the invention and the glass with nuclei accordingto the invention, wherein a correspondingly composed starting glass, theglass with nuclei according to the invention or the lithium metasilicateglass ceramic according to the invention is subjected to at least oneheat treatment in the range of from 450 to 950° C., in particular 450 to780 and preferably 480 to 750° C.

The starting glass according to the invention is therefore characterizedin that it comprises pentavalent metal oxide selected from Nb₂O₅, Ta₂O₅and mixtures thereof and comprises less than 2.0 wt.-% K₂O. In addition,it preferably also comprises suitable amounts of SiO₂ and Li₂O in orderto allow the formation of a lithium silicate glass ceramic, and inparticular a lithium disilicate glass ceramic. Further, the startingglass can also comprise still further components, such as are givenabove for the lithium silicate glass ceramic according to the invention.All those embodiments of the starting glass are preferred which are alsogiven as preferred for the glass ceramic.

With the process according to the invention, the glass with nuclei isusually prepared by means of a heat treatment of the starting glass at atemperature of in particular from 480 to 500° C. The lithium disilicateglass ceramic according to the invention is then preferably producedfrom the glass with nuclei through further heat treatment at usually 600to 750 and in particular 650 to 750° C.

Thus, much lower temperatures are used according to the invention forthe crystallization of lithium disilicate than with the conventionallithium disilicate glass ceramics. The energy thus saved represents aclear advantage. Surprisingly, this low crystallization temperature isalso possible in the absence of components, such as further alkali metaloxides and La₂O₃, regarded as essential for conventional glass ceramics.

To prepare the starting glass, the procedure is in particular that amixture of suitable starting materials, such as carbonates, oxides,phosphates and fluorides, is melted at temperatures of in particularfrom 1300 to 1600° C. for 2 to 10 h. To achieve a particularly highhomogeneity, the obtained glass melt is poured into water in order toform a granular glass material, and the obtained granular material isthen melted again.

The melt can then be poured into moulds to produce blanks of thestarting glass, so-called solid glass blanks or monolithic blanks.

It is also possible to put the melt into water again in order to preparea granular material. This granular material can then be pressed, aftergrinding and optionally addition of further components, such ascolourants and fluorescent agents, to form a blank, a so-called powdergreen compact.

Finally, the starting glass can also be processed to form a powder aftergranulation.

The starting glass, e.g. in the form of a solid glass blank, a powdergreen compact or in the form of a powder, is then subjected to at leastone heat treatment in the range of from 450 to 950° C. It is preferredthat a first heat treatment is initially carried out at a temperature inthe range of from 480 to 500° C. to prepare a glass according to theinvention with nuclei which are suitable for forming lithiummetasilicate and/or lithium disilicate crystals. This first heattreatment is preferably carried out for a period of from 10 min to 120min and in particular 10 min to 30 min. The glass with nuclei can thenpreferably be subjected to at least one further temperature treatment ata higher temperature and in particular more than 570° C. to effectcrystallization of lithium metasilicate or lithium disilicate. Thisfurther heat treatment is preferably carried out for a period of from 10min to 120 min, in particular 10 min to 60 min and particularlypreferably 10 min to 30 min. To crystallize lithium disilicate, thefurther heat treatment is usually carried out at 600 to 750 and inparticular 650 to 750° C.

Therefore, in a preferred embodiment of the process

-   -   (a) the starting glass is subjected to a heat treatment at a        temperature of from 480 to 500° C. in order to form the glass        with nuclei, and    -   (b) the glass with nuclei is subjected to a heat treatment at a        temperature of from 650 to 750° C. in order to form the glass        ceramic with lithium disilicate as main crystal phase.

The duration of the heat treatments carried out in (a) and (b) ispreferably as given above.

The at least one heat treatment carried out in the process according tothe invention can also take place during a hot pressing or sintering-onof the glass according to the invention or the glass ceramic accordingto the invention.

Dental restorations, such as bridges, inlays, onlays, crowns, veneers,facets or abutments, can be prepared from the glass ceramics accordingto the invention and the glasses according to the invention. Theinvention therefore also relates to their use for the preparation ofdental restorations. It is preferred that the glass ceramic or the glassis shaped into the desired dental restoration by pressing or machining.

The pressing is usually carried out at increased pressure and increasedtemperature. It is preferred that the pressing is carried out at atemperature of from 700 to 1200° C. It is further preferred to carry outthe pressing at a pressure of from 2 to 10 bar. During pressing, thedesired shape change is achieved by viscous flow of the material used.The starting glass according to the invention and in particular theglass with nuclei according to the invention, the lithium metasilicateglass ceramic according to the invention and the lithium disilicateglass ceramic according to the invention can be used for the pressing.The glasses and glass ceramics according to the invention can be used inparticular in the form of blanks, e.g. solid blanks or powder greencompacts, e.g. in unsintered, partly sintered or densely-sintered form.

The machining is usually carried out by material removing processes andin particular by milling and/or grinding. It is particularly preferredthat the machining is carried out as part of a CAD/CAM process. Thestarting glass according to the invention, the glass with nucleiaccording to the invention, the lithium metasilicate glass ceramicaccording to the invention and the lithium disilicate glass ceramicaccording to the invention can be used for the machining. The glassesand glass ceramics according to the invention can be used in particularin the form of blanks, e.g. solid blanks or powder green compacts, e.g.in unsintered, partly sintered or densely-sintered form. The lithiummetasilicate glass ceramic according to the invention and lithiumdisilicate glass ceramic according to the invention are preferably usedfor the machining. The lithium disilicate glass ceramic can also be usedin a not yet fully crystallized form which was produced by heattreatment at a lower temperature. This has the advantage that an easiermachining, and thus the use of simpler equipment for the machining, ispossible. After the machining of such a partly crystallized material,the latter is usually subjected to a heat treatment at a highertemperature and in particular about 650 to 750° C. in order to effectfurther crystallization of lithium disilicate.

In general, after the preparation of the dental restoration shaped asdesired by pressing or machining, the latter can also in particular beheat-treated in order to convert the precursors used, such as startingglass, glass with nuclei or lithium metasilicate glass ceramic, intolithium disilicate glass ceramic or to increase the crystallization oflithium disilicate or to reduce the porosity, e.g. of a porous powdergreen compact used.

However, the glass ceramic according to the invention and the glassaccording to the invention are also suitable as coating material of e.g.ceramics and glass ceramics. The invention is therefore also directed tothe use of the glass according to the invention or the glass ceramicaccording to the invention for coating of in particular ceramics andglass ceramics.

The invention also relates to a process for coating ceramics and glassceramics, wherein the glass ceramic according to the invention or theglass according to the invention is applied to the ceramic or glassceramic and is subjected to increased temperature.

This can take place in particular by sintering-on and preferably bypressing-on. With sintering-on, the glass ceramic or the glass isapplied to the material to be coated, such as ceramic or glass ceramic,in the usual way, e.g. as powder, and then sintered at increasedtemperature. With the preferred pressing-on, the glass ceramic accordingto the invention or the glass according to the invention is pressed on,e.g. in the form of powder green compacts or monolithic blanks, at anincreased temperature of e.g. from 700 to 1200° C., applying pressure,e.g. 2 to 10 bar. The methods described in EP 231 773 and the pressfurnace disclosed there can be used in particular for this. A suitablefurnace is e.g. the Programat EP 5000 from Ivoclar Vivadent AG,Liechtenstein.

It is preferred that, after conclusion of the coating process, the glassceramic according to the invention is present with lithium disilicate asmain crystal phase, as it has particularly good properties.

Because of the above-described properties of the glass ceramic accordingto the invention and the glass according to the invention as itsprecursor, these are suitable in particular for use in dentistry. Asubject of the invention is therefore also the use of the glass ceramicaccording to the invention or the glass according to the invention as adental material and in particular for the preparation of dentalrestorations or as a coating material for dental restorations, such ascrowns, bridges and abutments.

Finally, the glasses and glass ceramics according to the invention canalso be mixed together with other glasses and glass ceramics in order toproduce dental materials with properties adjusted as desired.Compositions and in particular dental materials which comprise the glassaccording to the invention or the glass ceramic according to theinvention in combination with at least one other glass and/or one otherglass ceramic therefore represent a further subject of the invention.The glass according to the invention or the glass ceramic according tothe invention can therefore be used in particular as main component ofan inorganic-inorganic composite or in combination with a plurality ofother glasses and/or glass ceramics, wherein the composites orcombinations can be used in particular as dental materials. Thecombinations or composites can particularly preferably be present in theform of sintered blanks. Examples of other glasses and glass ceramicsfor the preparation of inorganic-inorganic composites and ofcombinations are disclosed in DE 43 14 817, DE 44 23 793, DE 44 23 794,DE 44 28 839, DE 196 47 739, DE 197 25 553, DE 197 25 555, DE 100 31 431and DE 10 2007 011 337. These glasses and glass ceramics belong to thegroup of silicates, borates, phosphates or aluminosilicates. Preferredglasses and glass ceramics are of SiO₂—Al₂O₃—K₂O type (with cubic ortetragonal leucite crystals), SiO₂—B₂O₃—Na₂O type, alkali-silicate type,alkali-zinc-silicate type, silicophosphate type, SiO₂—ZrO₂ type and/orlithium-aluminosilicate type (with spodumene crystals). By mixing suchglasses or glass ceramics with the glasses and/or glass ceramicsaccording to the invention, for example the coefficient of thermalexpansion can be adjusted as desired in a broad range of from 6 to20·10⁻⁶ K⁻¹.

The invention is explained in more detail below by means of examples.

EXAMPLES Examples 1 to 9—Composition and Crystal Phases

A total of 9 glasses and glass ceramics according to the invention withthe composition given in Table I were prepared by melting correspondingstarting glasses followed by heat treatment for controlled nucleationand crystallization.

For this, the starting glasses weighing from 100 to 200 g were firstmelted from usual raw materials at 1400 to 1500° C., wherein the meltingwas very easily possible without formation of bubbles or streaks. Bypouring the starting glasses into water, glass frits were prepared whichwere then melted a second time at 1450 to 1550° C. for 1 to 3 h forhomogenization.

In the case of Examples 1 to 6 and 8 to 9, the obtained glass melts werethen poured into preheated moulds in order to produce glass monoliths.

In the case of Example 7, the obtained glass melt was cooled to 1400° C.and converted to a fine-particle granular material by pouring intowater. The granular material was dried and ground to a powder with aparticle size of <90 μm. This powder was moistened with some water andpressed to form a powder green compact at a pressing pressure of 20 MPa.

The glass monoliths (Examples 1-6 and 8-9) as well as the powder greencompact (Example 7) were then converted by thermal treatment to glassesand glass ceramics according to the invention. The thermal treatmentsused for controlled nucleation and controlled crystallization are alsogiven in Table I. The following meanings apply

-   -   T_(N) and t_(N) temperature and time used for nucleation    -   T_(C) and t_(C) temperature and time used for crystallization of        lithium disilicate    -   LS lithium metasilicate    -   LP lithium orthophosphate

It can be seen that a first heat treatment in the range of from 480 to500° C. resulted in the formation of lithium silicate glasses withnuclei and these glasses already crystallized due to a further heattreatment at 650 to 750° C. within only 20 min to glass ceramics withlithium disilicate as main crystal phase, as was established by X-raydiffraction tests.

The produced lithium disilicate glass ceramics had high fracturetoughness values, measured as critical stress intensity factor K_(IC),of up to more than 2.4 MPa·m^(0.5).

The produced lithium disilicate glass ceramics were able to be verysatisfactorily machined into the form of various dental restorations, ina CAD/CAM process or by hot pressing, which restorations were alsoprovided with a veneer if required.

They were also able to be applied by hot pressing as coatings onto inparticular dental restorations, e.g. in order to veneer the latter asdesired.

Example 10—Processing Via Powder Green Compacts

The glass ceramics according to Examples 1 and 4 were ground to powderswith an average particle size of <90 μm.

In a first variant, the obtained powders were pressed with or withoutpressing auxiliaries to powder green compacts and the latter were partlyor densely sintered at temperatures of from 800 to 1100° C. and thenfurther processed by machining or by hot pressing to form dentalrestorations.

In a second variant, the obtained powders were pressed with or withoutpressing auxiliaries to powder green compacts and the latter were thenfurther processed by machining or by hot pressing to form dentalrestorations. In particular, the dental restorations obtained after themachining were then densely sintered at temperatures of from 900 to1100° C.

With both variants, in particular crowns, caps, partial crowns andinlays as well as coatings on dental ceramics and dental glass ceramicswere prepared.

Example 11—Hot Pressing of Glass with Nuclei

A glass with the composition according to Example 7 was prepared bymixing corresponding raw materials in the form of oxides and carbonatesfor 30 min in a Turbula mixer and then melting the mixture at 1450° C.for 120 min in a platinum crucible. The melt was poured into water inorder to obtain a fine-particle granular glass material. This granularglass material was melted again at 1530° C. for 150 min in order toobtain a glass melt with particularly high homogeneity. The temperaturewas reduced to 1500° C. for 30 min and cylindrical glass blanks with adiameter of 12.5 mm were then poured into pre-heated, separable steelmoulds or graphite moulds. The obtained glass cylinders were thennucleated at 490° C. and stress-relieved.

The nucleated glass cylinders were then processed by hot pressing at apressing temperature of 970° C. and a pressing time of 6 min using anEP600 press furnace, Ivoclar Vivadent AG, to form dental restorations,such as inlays, onlays, veneers, partial crowns, crowns, laminatingmaterials and laminates. In each case, lithium disilicate was detectedas main crystal phase.

TABLE I Example 1 2 3 4 5 6 7 8 9 Composition wt.-% wt.-% wt.-% wt.-%wt.-% wt.-% wt.-% wt.-% wt.-% SiO₂ 73.8 73.8 69.4 73.8 76.4 73.8 76.376.25 66.3 Li₂O 15.3 15.3 19.7 15.3 12.7 15.3 15.9 15.9 18.9 P₂O₅ 3.43.4 3.4 3.4 3.4 7.0 — — 3.2 Al₂O₃ 3.5 — 3.5 3.5 3.5 — 3.6 3.6 3.4 ZrO₂ —3.5 — — — — — — — Nb₂O₅ 4.0 — 4.0 2.0 4.0 — 2.1 2.1 4.1 Ta₂O₅ — 4.0 —2.0 — 3.9 2.1 2.1 4.1 Pd — — — — — — — 0.05 — SiO₂/Li₂O molar 2.39 2.391.75 2.39 3.00 2.39 2.39 2.39 1.75 ratio Optical properties trans-trans- trans- trans- trans- opaque trans- trans- trans- (after pouring)parent parent parent parent parent glass parent parent parent T_(g)/° C.480 476 461 477 477 466 471 481 466 T_(N)/° C. 500 500 480 500 500 490490 500 490 t_(N)/min. 10 10 10 10 10 10 10 10 10 T_(C)/° C. 700 750 700680 650 750 750 750 720 t_(C)/min. 20 20 20 20 20 20 20 20 20 Maincrystal lithium lithium lithium lithium lithium lithium lithium lithiumlithium phase_(RT-XRD) disilicate disilicate disilicate disilicatedisilicate disilicate disilicate disilicate disilicate Other crystal LPLS, LP LS, LP LP — LS, LP LS, quartz — LS, LP phases K_(IC)/MPa ·m^(1/2) — 2.49 2.48 — — — — — —

1. Lithium silicate glass ceramic which comprises a pentavalent metaloxide selected from Nb₂O₅, Ta₂O₅ and mixtures thereof and comprises lessthan 2.0 wt.-% K₂O.
 2. Glass ceramic according to claim 1, whereinlithium silicate glass ceramic is excluded which comprises at least 6.1wt.-% ZrO₂.
 3. Glass ceramic according to claim 1, wherein glass ceramicis excluded which comprises at least 8.5 wt.-% transition metal oxideselected from the group consisting of oxides of yttrium, oxides oftransition metals with an atomic number from 41 to 79 and mixtures ofthese oxides.
 4. Glass ceramic according to claim 1, which comprisesless than 1.0 wt.-% K₂O.
 5. Glass ceramic according to claim 1, whichcomprises less than 1.0 wt.-% Na₂O.
 6. Glass ceramic according to claim1, which comprises at least 2.0 wt.-% Nb₂O₅ or Ta₂O₅.
 7. Glass ceramicaccording to claim 1, which comprises less than 0.1 wt.-% La₂O₃. 8.Glass ceramic according to claim 1, which comprises the pentavalentmetal oxide or mixtures thereof in an amount of from 0.1 to 8.2 wt.-%.9. Glass ceramic according to claim 1, which has lithium metasilicate asmain crystal phase.
 10. Glass ceramic according to claim 1, which haslithium disilicate as main crystal phase.
 11. Glass ceramic according toclaim 1, which comprises 60.0 to 85.0 wt.-% SiO₂.
 12. Glass ceramicaccording to claim 1, which comprises 11.0 to 21.0 wt.-% Li₂O.
 13. Glassceramic according to claim 1, which comprises 0 to 10.0 wt.-% P₂O₅. 14.Glass ceramic according to claim 1, which comprises at least one andpreferably all of the following components: Component wt.-% SiO₂ 66.0 to77.0 Li₂O 12.0 to 20.0 pentavalent metal 2.0 to 8.5 oxide or mixturesP₂O₅ 0 to 7.0 Al₂O₃ 0 to 6.0


15. Lithium silicate glass ceramic according to claim 1, which haslithium disilicate as main crystal phase and a fracture toughness,measured as K_(IC) value, of at least 1.6 MPa·m^(0.5).
 16. Lithiumsilicate glass ceramic according to claim 1, which comprises SiO₂ andLi₂O in a molar ratio of from 1.7 to 3.1.
 17. Lithium silicate glassceramic according to claim 1, which comprises less than 0.5 wt.-% CaO.18. Lithium silicate glass ceramic according to claim 1, which comprisesless than 0.5 wt.-% B₂O₃.
 19. Starting glass, which comprises thecomponents of the glass ceramic according to claim
 1. 20. Lithiumsilicate glass with nuclei which are suitable for forming lithiummetasilicate and/or lithium disilicate crystals, wherein the glasscomprises the components of the glass ceramic according to claim
 1. 21.Glass ceramic according to claim 1, wherein the glass ceramic is presentin the form of a powder, a granular material, a blank or a dentalrestoration.
 22. Process for the preparation of the glass ceramicaccording to claim 1, wherein a starting glass, a glass with nuclei, orthe glass ceramic with lithium metasilicate as a main crystal phase issubjected to at least one heat treatment in the range of from 450 to950° C.
 23. Process according to claim 22, wherein (a) the startingglass is subjected to a heat treatment at a temperature of from 480 to500° C. in order to form the glass with nuclei, and (b) the glass withnuclei is subjected to a heat treatment at a temperature of from 650 to750° C. in order to form the glass ceramic with lithium disilicate asmain crystal phase.
 24. Method of using the glass ceramic according toclaim 1 as dental material for the preparation of dental restorations.25. Method of preparing the dental restorations according to claim 24,wherein the glass ceramic or a glass which comprises the components ofthe glass ceramic is shaped by pressing or machining to the desireddental restoration.
 26. Glass ceramic according to claim 1, which haslithium metasilicate as a main crystal phase and has more than 5 vol.-%,lithium metasilicate crystals.
 27. Glass ceramic according to claim 1,which has lithium disilicate as a main crystal phase and has more than10 vol.-% lithium disilicate crystals.
 28. Glass according to claim 19,wherein the glass is present in the form of a powder, a granularmaterial, a blank or a dental restoration
 29. Process for thepreparation of the glass according to claim 20, wherein a starting glassor a glass with nuclei which are suitable for forming lithiummetasilicate and/or lithium disilicate crystals is subjected to at leastone heat treatment in the range of from 450 to 950° C.
 30. Processaccording to claim 29, wherein (a) the starting glass is subjected to aheat treatment at a temperature of from 480 to 510° C. in order to formthe glass with nuclei, and (b) the glass with nuclei is subjected to aheat treatment at a temperature of from 630 to 720° C. in order to formthe glass ceramic with lithium disilicate as main crystal phase. 31.Method of using the glass according to claim 19 as dental material forthe preparation of dental restorations.
 32. Method of preparing thedental restorations according to claim 31, wherein the glass is shapedby pressing or machining to the desired dental restoration.